Such RLG systems typically include a signal generator, means to emit the signal into the tank, and a receiver for receiving the reflected tank signal. The received signal can for example be a time domain reflectometry (TDR) signal or a frequency domain signal, such as a frequency modulated continuous wave (FMCW) signal. The received signal typically comprises at least a surface reflection (echo) caused by an interface between different materials in the tank, typically but not necessarily a liquid surface. Normally, the received signal also includes various interfering reflections caused e.g. by the bottom and walls of the tank or the transition between the signal generator and the wave guide.
In order to improve the accuracy of the measurement result, the signal processing of the received signal can be adapted to compensate the received signal for such interfering reflections. However, the signal processing is typically optimized in terms of general precision in the entire tank, and is not necessarily optimal in all areas of the tank. Therefore, in addition to the RLG system, additional sensors are sometimes arranged in the tank, in order to provide information about conditions in specific regions of the tank.
One such region of special importance is the near zone, i.e. in a region close to the entry of microwaves into the tank. In an RLG, the measurement process can be complicated or even made impossible, when the surface reflection occurs in this near zone, which can be in the range of 0 m to 2 m depending on the type of microwave signal used. The problems are caused by an interfering reflection caused by the transition between a signal transfer medium and the emitter/receiver in the tank, in combination with the limited bandwidth limiting the resolution. RLG systems may be provided with specific signal processing to handle such problems.
At the same time, for security reasons it is very important to have a secure indication of if and when the surface of the contents in the tank approaches the top of the tank, i.e. some kind of overfill sensing system. Therefore, one example of an additional sensor mentioned above is an overfill sensor, arranged in the top of the tank and adapted to detect when the level exceeds a certain level. Although the normal measurement processing may be adapted to provide accurate measurements also in the top of the tank (referred to as the near zone), such a redundant sensor system is required by authorities in many countries.